At $4.95 per month, the product is aiming for a low cost. But the addition of the pricing tier represents an important move for 410 Labs, which has so far built apps and web-based services by sustaining itself on funding, which includes an equity round of nearly $800,000 raised in 2011.

“From the beginning we have been focused on building useful services that can grow to a large enough scale that we can monetize them meaningfully,” said 410 Labs cofounder Dave Troy in an email. “Mailstrom has met that criteria, and is on course to be a very successful product and the basis of a strong business.”

Troy said that based on testing 410 Labs conducted in fall 2013, he expects that 18 percent of people who try Mailstrom will become paid subscribers. (Right now Mailstrom counts 80,000 registered users. If 18 percent of that number signed up solely by the month, that would be a little more than $71,000 in monthly revenue.)

He also said that 410 Labs is “in the process of finalizing an additional modest funding round to expand the Mailstrom business this year.”

Andrew Zaleski is a freelance journalist in Philadelphia and the former lead reporter for Technical.ly Baltimore. Before moving to Philadelphia in June 2014, he was a contributing writer to Baltimore City Paper and a Tech Check commentator for WYPR 88.1 FM, Baltimore city’s National Public Radio affiliate. He has written for The Atlantic, Outside, Richmond magazine, Washington City Paper, Baltimore magazine, Baltimore Style magazine, Next City, Grist.org, The Atlantic Cities, and elsewhere.